We are in the process of studying the physiological, structural and ultrastructural response of cardiac muscle to hyperosmolar media. Using a microelectrode recording technique we plan to evaluate the basic for the increase in rate of spontaneously beating atria in hyperosmolar media. We have found that the increase in passive tension of cardiac muscle in hyperosmolar media is correlated with the appearace of hypercontracted sarcomeres and regions of widened or hypertrophied Z bands. We plan to compare the degree of structural change at the light microscopic level with the observed increase in passive tension. On return to normal Tyrode solution tension slowly decrease toward the initial baseline level. We plan to determine whether this decrease in tension represents a reversal of contracture and return to normal myofibrilar configuration or whether a breakdown of myofilaments is involved as appears likely from our earlier EM study.